1. Field of the Invention
The present invention relates to a laser crystallization apparatus which generates a crystallized semiconductor film by irradiating a polycrystal semiconductor film or an amorphous semiconductor film with a laser light having a predetermined light intensity distribution.
2. Description of the Related Art
A thin film transistor (TFT) used for a switching element or the like which selects a display pixel in a liquid crystal display (LCD) is formed by using amorphous silicon or polysilicon.
Polysilicon has a higher mobility of electrons or electron holes than that of amorphous silicon. Therefore, when a transistor is formed by using polysilicon, a switching speed and hence a display response speed become higher than those in the case of forming the same by using amorphous silicon. Further, a peripheral LSI can comprise a thin film transistor formed on glass. Furthermore, there is an advantage of reducing a design margin of any other component. Moreover, when peripheral circuits such as a driver circuit or a DAC are incorporated in a display, these peripheral circuits can be operated at a higher speed.
Since polysilicon comprises an aggregation of crystal grains, when, e.g., a TFT transistor is formed, a crystal grain boundary is formed in a channel region, this crystal grain boundary serves as a barrier, and a mobility of electrons or electron holes is reduced as compared with that of single-crystal silicon. Additionally, each of many thin film transistors formed by using polysilicon has a different number of crystal grain boundaries formed in a channel region, and this difference becomes an irregularity, resulting in a problem of ununiformity in display in the case of LCD. Thus, there has been recently proposed a larger induced crystallization method which generates crystallized silicon having a larger grain size than with the channel region variety in number of crystal grain boundaries in the channel region.
As this type of crystallization method, there is known a “phase control ELA (Excimer Laser Annealing) method” which generates a crystallized semiconductor film by irradiating a phase shifter approximated in parallel with a polycrystal semiconductor film or a non-single-crystal semiconductor film with an excimer laser light. The details of the phase control ELA method is disclosed in, e.g., Journal of The Surface Science Society of Japan, Vol. 21, No. 5, pp. 278-287, 2000.
In the phase control ELA method, a light intensity distribution having an inverse peak pattern (a pattern in which a light intensity is minimum at the center and the light intensity is suddenly increased toward the periphery) in which a light intensity at a point corresponding to a phase shift portion of a phase shifter is lower than that in the periphery is generated, and a non-single-crystal semiconductor film (a polycrystal semiconductor film or an amorphous semiconductor film) is irradiated with a light having this light intensity distribution with an inverse peak shape. As a result, a temperature gradient is generated in a fusion area in accordance with a light intensity distribution in an irradiation target area, a crystal nucleus is formed at a part which is solidified first or a part which is not fused in accordance with a point where the light intensity is minimum, and a crystal grows from the crystal nucleus in a lateral direction toward the periphery, thereby generating a single-crystal grain with a large particle size.
Further, in Jpn. Pat. Appln. KOKAI Publication No. 2000-306859, an image formation optical system is arranged between a phase shifter having a line-and-space pattern with a phase difference of 180 degrees and a processed substrate. Furthermore, the processed substrate is irradiated through the image formation optical system with a light having a light intensity distribution with an inverse peak pattern generated through the phase shifter, thereby generating a crystallized semiconductor film on the processed substrate.